A planet outside of our solar system. It might orbit a star, or it might be a rogue planet traveling through the galaxy alone. There are estimated to be hundreds of billions of stars in our Milky Way galaxy and each is likely to have at least one planet orbiting it. If this is the case, there could be well over one trillion planets in this galaxy alone.

The first confirmed exoplanet orbiting a star like our own was discovered in 1995, and since then, thousands more have been observed. To date, NASA has cataloged over 4,000 confirmed exoplanets in its Exoplanet Archive. Their size, orbit, and composition vary greatly.

Exoplanets are categorized just like the planets of the solar system. There are terrestrial exoplanets, small and rocky like Earth, and gas giants like Jupiter. Their size and composition vary. We describe them relative primarily to Earth and Jupiter: large terrestrial exoplanets between the size of Earth and Neptune are super-Earths. Some exoplanets are tidally locked, with one side always facing their stars, while others may rotate.

Their scientific nomenclature has multiple components. Exoplanet Kepler-16b is one example: first, it is named after its host, Kepler-16, a binary star system discovered by the Kepler telescope. Second, the lowercase “b” denotes that it is a planet instead of a star – Kepler-16B is one of the stars in the system. Other exoplanets and systems may be named after the project that discovered them (for example, Corot-7b) or after their star’s common name (for example, 51 Pegasi b).

Planets outside of the solar system have been theorized and discussed throughout history, as has the possibility of and search for extraterrestrial life. Finding evidence of current life outside of Earth is one of NASA’s biggest scientific goals. For hundreds, if not thousands of years, humans have looked up at the sky wondering about what could be beyond our planet. Beliefs about the structure and nature of the universe have varied, but now we know – and can see – that there are planets outside of the solar system, many in habitable zones and possibly harboring life.

The search for life and ideas of how life could form on an exoplanet are based on Earth since it is the only planet known to host life. The “habitable zone” that some exoplanets are determined to be in is also called the “Goldilocks zone”: an orbital range in which the planet is not too close to its star and not too far, allowing for temperatures at which liquid water could exist. Planetary composition is also important: terrestrial planets are the best possibility for life.

Although exoplanets may be hundreds or thousands of light-years away from Earth, their characteristics can be determined through a variety of methods. The majority of these planets are found indirectly as it is rare for an exoplanet to be discovered through direct imaging, where it is directly observed. Observation is tricky due to several different factors, such as the enormous distance from Earth and the extreme brightness and much larger size of the parent stars dwarfing them. Because of this, a few different methods have been developed.